1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus including the sheet conveying device.
2. Description of the Related Art
An image forming apparatus in an electrophotographic system exposes and scans a surface of a photoreceptor based on image data of a document to form an electrostatic latent image, supplies a toner to the electrostatic latent image to generate a toner image, transfers the toner image on a recording sheet, and then thermally fixes the toner image in a fixing device.
Typically, the fixing device has a configuration of passing a recording sheet to a nip portion formed between a heated fixing roller (fixing rotating body) and a pressure roller (pressure rotating body) pressed against the fixing roller, and conveying the sheet forward while thermally fixing the sheet. The recording sheet adheres to a surface of the fixing roller by adhesive force of a toner heated in the nip portion and becomes in a fused state. Accordingly, the recording sheet is not separated from the fixing roller, and a jam (separation jam) occurs. In a worst case, the recording sheet fully winds around the fixing roller, and maintenance becomes an enormous burden.
Therefore, a configuration of forming a mold release layer mainly made of a fluororesin on the surface of the fixing roller to make the recording sheet be easily separated is employed, and it is devised to cause the recording sheet to be able to be separated by itself due to a curvature of a peripheral surface of the fixing roller, and firmness of the recording sheet itself (curvature separation).
Nonetheless, when a sheet without having firmness such as a thin paper or a thin-paper-coated sheet is used as the recording sheet, or when there is a large amount of adhesion of a toner to the recording sheet, such as a solid image, a possibility of occurrence of the separation jam may become high.
To solve such a problem, a method (separating claw system) of bringing a separating claw to come in contact with the peripheral surface of the fixing roller to forcibly separate the recording sheet from the fixing roller can be considered, for example. However, if the separating claw is in contact with the peripheral surface of the fixing roller on a steady basis, a portion of the peripheral surface of the fixing roller, the portion being contact with the separation claw, is worn away, and nonuniformity may occur in a fixing property. As a forcible sheet separating mechanism, there is a method of separating the recording sheet without generating scratches in the peripheral surface of the fixing roller, by strongly blowing air from a tip of a nozzle to a gap slightly caused between a tip of the recording sheet and the periphery surface of the fixing roller according to the curvature separation, and forcibly separating the recording sheet in a non-contact manner, in place of the separating claw system (hereinafter, may be referred to as “air type separation system”). However, heat is taken from the fixing roller by the air flow on a steady basis. Therefore, there is a problem of an increase in power consumption for maintaining the fixing roller to have a fixing temperature.
Therefore, for example, JP 2007-108618 A operates the sheet separating mechanism only when a separation state between the recording sheet and the fixing roller is defective while adopting the separating claw system.
That is, as illustrated in FIG. 24A, a configuration of forcibly separating a recording sheet S, by measuring a distance to a tip portion of the recording sheet S, which has passed a nip portion N formed between a fixing roller 543 and a pressure roller 544, with a laser displacement sensor 545, to calculate a winding index value, determining that a separation property is deteriorated due to expiration of life of the fixing roller 543 when the winding index value becomes larger than a predetermined value, and operating the sheet separating mechanism, for sheet passing on and after the recording sheet S while issuing a warning to a user.
In this sheet separating mechanism 560, a holding member 562, which holds the separating claw 563, is pivotally fixed to a main body frame (not illustrated) of the fixing device with a support shaft 562a at a base end part of the sheet separating mechanism 560 in a swingable manner. The holding member 562 is energized to a direction of being in contact with a cam plate 561 by a spring member (not illustrated), and when the separation state between the recording sheet and the fixing roller is determined to be deteriorated, the cam 561 is rotated by a drive source such as a motor, so that the holding member 562 swings in the arrow direction, and a tip of the separating claw 563 comes in contact with the peripheral surface of the fixing roller 543.
If the separation state of the recording sheet is determined using such a technology, and the sheet separating mechanism is operated only when the separation state is not favorable, the degree of the flaw occurring in the peripheral surface of the fixing roller can be substantially decreased and durability can be improved, in the case of the separating claw system. There is a high possibility of avoiding the inconvenience that the heat is taken from the fixing roller on a steady basis and the power consumption is increased, in the case where the sheet separating mechanism is the air type separation system.
By the way, the laser displacement sensor used for determination of the sheet separation state in JP 2007-108618 A has a configuration of detecting displacement of a spot position of reflection light of laser light from the recording sheet surface to measure a distance. Therefore, to accurately detect the distance, it is desirable to arrange the laser displacement sensor in a position where an amount of the reflection light from a body to be detected (the recording sheet) is large. Therefore, it is necessary to irradiate the recording sheet surface with the laser light from an angle as nearly perpendicular as possible to the recording sheet surface (condition a).
Further, to determine the separation state between the recording sheet and the fixing roller 543, it is desirable to measure a state of the tip of the recording sheet immediately after the tip has passed the nip portion (condition b).
However, if an incident angle of the laser to the recording sheet is brought to achieve a right angle, and the measurement is performed with the laser displacement sensor 545 of the recording sheet near the nip portion, in order to satisfy both of the conditions a and b, the laser displacement sensor 545 interferes with the pressure roller 544, like a laser displacement sensor 545′ illustrated by the two-dotted chain line of FIG. 24B. In a real sense, installation is difficult.
Therefore, as illustrated in FIG. 24A, in JP 2007-108618 A, the laser displacement sensor 545 is forced to be arranged slightly diagonally forward in an advancing direction of the recording sheet. In this case, the position of the tip of the recording sheet immediately after the tip of the recording sheet has passed the nip portion cannot be accurately measured. As a result, determination accuracy of the separation state becomes worse and a determination time becomes late.
Especially, in recent years, a conveying speed of the recording sheet is increased in order to improve productivity. If the determination of the defectiveness of the separation of the recording sheet is late, the operation of the sheet separating mechanism cannot be in time, and there is a high possibility that the separation jam and winding of the recording sheet to the fixing roller cannot be avoided in advance.
The problems of the separation jam and the winding are caused not only in the nip portion between the fixing roller and the pressure roller, but also in a case where some sort of adhesive force occurs between the sheet surface as an object to be conveyed and the surface of the rotating body, in a configuration in which the sheet is conveyed by being passed to the nip portion of a pair of rotating bodies. Therefore, a technology to accurately detect the separation state between the sheet and the surface of the rotating body is desired.